水淹干扰对羊草草地地上生物量影响的初步研究

为在一定程度上揭示水淹干扰后草地净初级生产力变化的机制，对松嫩平原羊草草地水淹干扰梯度上的地上生物量进行了测定和比较，并对经历水淹干扰后土壤水分及主要养分(N、P)的变化以及植被物种组成的变化作了比较分析。结果表明，轻度和重度水淹干扰样带的地上生物量显著高于未受水淹干扰的对照样带，分别高出对照样带89．54％和113.45％，表明水淹干扰消除了对草地生产力起限制作用的因素，使草地净初级生产力有了大幅度提高，水淹干扰首先改变了土壤的水分状况，而水分状况是限制草地生产力的最主要的因素，尤其是在干旱年份，消除了干旱对草地生产力的限制；土壤水分的增加导致土壤养分(N、P等)的有效性显著增加，消除了原来土壤养分匮乏对草地生产力造成的限制；在水淹干扰作用下，群落的物种组成由相对低矮的物种组合趋向于向形态高大的、具有更高生产力潜力的物种组合转变。

Effect of flooding disturbance on aboveground biom ass of Leymus chinensis grassland- A preliminary study

To investigate the effect of flooding disturbance on the net primary productivity of Songnen steppe, a comparatively thorough study was conducted on Sanjiadian State-owned Rangeland in Da'an city, Jilin Province, which was partly flooded in 1998. The study site was located in the south Songnen plain of Northeastern China, dominated by Leymus chinensis grassland. An extensively mild slope with flooding gradients (from un-flooded to heavily flooded) was taken as the study site. Two flooded transects coded FL and FH which was respectively subjected to 3 and 9 months of flooding were designed, and an un-flooded one coded CK at a relatively higher elevation was set as a control. Before flooding occurred in 1998, the slope had an almost uniform soil and L. chinensis dominated vegetation. Each transect was 0.2 hm2 (100 m x 20 m) in size, and the two flooded transects were almost paralleled each other, with the longer sides of them perpendicular to the retrieving direction of floodwater. In each transect twenty 1 m2 sized quadrats were randomly chosen to survey the community structure and the aboveground biomass. Comparative analyses were made on the dynamics of soil water, soil N and P, and species composition of grassland communities that occurred in responses to flooding disturbance. The results showed that the lightly and heavily flooded transects had a significantly larger aboveground biomass than the control, with the increase of 89.54% and 113.45%, respectively. The heavily flooded transect had a slightly but insignificantly larger aboveground biomass than the lightly flooded one, indicating that on flooded sites, water was not the limiting factor of the aboveground biomass. The acute changes of soil water caused by flooding led to the changes of soil nutrients and species assemblages, which would impact community biomass. Just as the case for aboveground biomass, the soil water contents of the two flooded transects were significantly larger than that of control, which was the direct effect of flooding disturbance. The contents of soil available N and P on the flooded transects were higher than that on the control, as resulted by the increase of soil water. The species composition had an obvious difference between flooded transects and control, and the major change on the flooded transects was the decrease of Xeric and Mesoxeric plants and the increase of Hygric and Hygro-mesic plants, compared to the control. It's deduced from the results that the mechanisms that the flooding disturbance influenced the net primary productivity were: 1) Flooding disturbance firstly changed soil water content, the leading limiting factor of grassland productivity, especially in dry years, and thus, lessened the limitation of aridity on grassland productivity. 2) Flooding induced increase of soil water content led to the drastic increases of nutrient availability, and thereby, eliminated the restriction of pre-flood nutrient scarcity on grassland productivity. 3) Still due to the afterward effects of flooding disturbance, the species composition of the community transformed from relatively shorter plant species assemblage towards taller and bigger plant species assemblage that were more productive.